This invention relates to a driving device (a linear actuator) and, in particular, to a driving device where an electro-mechanical transducer such a piezoelectric element is used as a driving source of the driving device (the linear actuator).
Previously, linear actuators (driving devices) using electro-mechanical transducers such as piezoelectric elements, electrostrictive elements, magnetostrictive elements, or the like are used as auto-focus actuators or zoom actuators for use in cameras.
By way of illustration, Japanese Patent No. 2633066 (JP-B 2633066) (which will be also called a first patent document), which corresponds to U.S. Pat. No. 5,225,941, discloses a driving device comprising a driving rod frictionally-engaged with a lens barrel, a piezoelectric element disposed in contact with the driving rod, and a leaf spring for bringing the driving rod into frictional engagement with the lens barrel. That is, the driving rod is bonded to an end of the piezoelectric element in an expansion direction. The lens barrel is movably supported to the driving rod. The leaf spring produces friction between the driving rod and the lens barrel. In the driving device disclosed in JP-B 2633066, a voltage is applied to the piezoelectric element so as to make a speed of expansion of the piezoelectric element different from a speed of contraction thereof.
In addition, Japanese Patent No. 3218851 (JP-B 3218851) (which will be also called a second patent document), which corresponds to U.S. Pat. No. 5,589,723, discloses a driving apparatus comprising a piezoelectric element, a driving member (a driving shaft), coupled to the piezoelectric element, for extending in an expansion direction of the piezoelectric element, and a driven member (a moving member, a lens barrel) having a friction member frictionally coupled to the driving member (the driving shaft). The driving apparatus in JP-B 3218851 drives the lens barrel by devising a driving signal applied to the piezoelectric element. Japanese Patent No. 3180557 (JP-B 3180557) (which will be also called a third patent document), which corresponds also to U.S. Pat. No. 5,589,723, discloses a driving apparatus including the friction member composed of a metal and the driving member made of a fiber-reinforced resin composite.
Japanese Unexamined Patent Application Publication No. 2006-54979 (JP-A 2006-54979) (which will be also called a fourth patent document) discloses an actuator which is capable of moving a driven member with stability and with precision. The actuator disclosed in JP-A 2006-54979 comprises a first piezoelectric element, a diving member mounted to the first piezoelectric element, a driven member which is frictionally coupled to the driving member and which extends in a driving direction, and a second piezoelectric element for engaging the driving member with the driven member and for releasing a friction engagement between the driving member and the driven member. Specifically, the actuator disclosed in JP-A 2006-54979 comprises a pair of first piezoelectric elements for driving, a second piezoelectric element for engagement, a pair of driving members, and a pressure spring. The pair of first piezoelectric elements and the pair of driving members are disposed at both sides with the driven member sandwiched between them. The pair of driving members have a distance which is expanded or narrowed by the second piezoelectric element. By the second piezoelectric element, the friction engagement between the pair of the driving members and the driven member is ensured or released.
Japanese Unexamined Patent Application Publication No. H9-191665 (JP-A 9-191665) (which will be also called a fifth patent document), which corresponds to U.S. Pat. No. 5,890,391, discloses a linear drive mechanism using an electro-mechanical conversion element which is insensitive to elastic deformation of a constituent member. The linear driving mechanism disclosed in JP-A 9-191665 comprises the electro-mechanical conversion element, a working member which is fixedly connected to the electro-mechanical conversion element and which displaces with the electro-mechanical conversion element, a driving member frictionally coupled to the working member, a driven member coupled to the driving member, and a driving pulse generating arrangement for causing expansion/contraction displacement to the electro-mechanical conversion element.
Japanese Unexamined Patent Application Publication No. 2006-304529 (JP-A 2006-304529) (which will be also called a sixth patent document) discloses a high-performance drive device whose cost and weight can be reduced in comparison with a structure with a metal material, and for which a moving member of high rigidity is used without deteriorating the speed of movement and a drive force. In the drive device disclosed in JP-A 2006-304529, the moving member is formed with a liquid crystal polymer including carbon fibers. The drive device disclosed in JP-A 2006-304529 comprises a piezoelectric element for expanding and contracting in response to an applied voltage, a drive shaft (a vibration friction portion) fixed to one end of the piezoelectric element in an expansion/contraction direction, the moving member (a moving portion) which is slidably and frictionally engaged with the drive shaft, and a weight (a stationary member) connected to another end of the piezoelectric element in the expansion/contraction direction. By vibrating the drive shaft by changing a speed or an acceleration of the piezoelectric element in the expansion/contraction direction, the drive device moves the moving member (the moving portion) along the drive shaft (the vibration friction portion). The drive shaft comprises a rod shaft body extending in a linear fashion. The moving member comprises a moving member body and a cap which are engaged with the drive shaft so that both sandwich the drive shaft. In the manner to be movable the moving member along the drive shaft, a substantially U-shaped leaf spring presses the moving member body and the cap against the drive shaft so that a predetermined friction force produces between the drive shaft and the moving member. The moving member body has a groove having a V-shaped cross section. The drive shaft fits in the groove of the moving member body to that two oblique surfaces of the groove come into contact with the drive shaft. Likewise, the cap has a groove having a V-shaped cross section. When the cap is assembled to the moving element body, the drive shaft fits in the groove of the cap so that two oblique surfaces of the groove come into contact with the drive shaft.
Furthermore, Japanese Patent No. 3141714 (JP-B 3141714) (which will be also called a seventh patent-document), which corresponds also to U.S. Pat. No. 5,589,723, discloses a driving device apparatus which is capable of driving a moving member with stability and at high-speed. The driving apparatus disclosed in JP-B 3141714 comprises a stationary member, an electro-mechanical transducer, a driving member (a vibration friction portion), a moving member (a moving portion), and a friction adding arrangement (a plate spring). The electro-mechanical transducer has one end in an expansion/contraction direction that is fixed to the stationary member. The electro-mechanical transducer is applied with a voltage so that the electro-mechanical transducer expands and contracts at different speeds on expanding and on contracting, respectively. The driving member (the vibration friction portion) is connected to another end of the electro-mechanical transducer and is supported so as to move in the expansion/contraction direction of the electro-mechanical transducer. The moving member is frictionally engaged with the driving member and is supported so as to move in the expansion/contraction direction of the electro-mechanical transducer. The friction adding arrangement provides a friction force between the driving member and the moving member. The friction force adding arrangement comprises an elastic member, fixed to the moving member (the moving portion), for generating a pressing force and a sandwiched member (a friction member) for transmitting the pressing force generated by the elastic member to the driving member. In addition, each of a contact portion between the moving member and the driving member and a contact portion between the moving member and the sandwiched member (the friction member) has a V-shaped cross section.
There are problems in the above-mentioned first through forth patent documents as follows.
In the driving device disclosed in the first patent document, inasmuch as the lens holder (the driven member, the moving portion) is movably supported to the guide bar (the driving member, the vibration friction portion) bonded to the piezoelectric element, the guide bar (the driving member, the vibration friction portion) has a length longer than that of the lens holder (the driven member, the moving portion) and the guide bar (the driving member, the vibration portion) is easy to produce an inclination caused by a reciprocating motion. In addition, the longer a moving distance of the lens holder (the driven member, the moving portion), the longer the guide bar (the driving member, the vibration friction member) and it results in easily producing an unnecessary vibration mode. Furthermore, inasmuch as the friction engagement portion lies on an extension of a coupling portion between the piezoelectric element and the guide bar (the driving member, the vibration friction member), it counts against a reduction in profile thereof.
In also the driving apparatus disclosed in the second patent document, inasmuch as the driving shaft (the driving member, the vibration friction portion) extends in an expansion/contraction direction of the electro-mechanical transducer, the driving shaft (the driving member, the vibration friction portion) has a length longer than that of the a zoom lens barrel (the driven member, the moving portion) and the driving shaft (the driving member, the vibration friction portion) is easy to produce an inclination caused by a reciprocating motion. In addition, the longer a moving distance of the zoom lens barrel (the driven member, the moving portion), the longer the driving shaft (the driving member, the vibration friction portion) and it results in easily producing an unnecessary vibration mode. Furthermore, inasmuch as the friction engagement portion lies on an extension of a coupling portion between the electro-mechanical transducer and the driving shaft (the driving member, the vibration friction member), it counts against a reduction in profile thereof. In addition, inasmuch as the driving apparatus has structure where the zoom lens barrel (the driven member, the moving portion) is cantilevered by the driving shaft (the driving member, the vibration friction portion), it is impossible to mechanically move a large weight object such as the lens.
The third patent document may just disclose the driving apparatus where the driving shaft (the driving member, the vibration friction portion) is made of the fiber-reinforced resin composite and has a basic structure which is similar to that illustrated in the second patent document. It has therefore disadvantage which is similar to that of the second patent document.
Although the driving member (the vibration friction portion) has a length which is shorter than that of the driven member (the moving portion) in the actuator disclosed in the fourth patent document, the actuator disclosed in the fourth patent document is disadvantageous in that it is complicated in structure and it is impossible to reduce a size thereof because it comprises a plurality of piezoelectric elements.
On the other hand, the fifth patent document discloses the linear drive mechanism where the moving member (the moving portion) has a rod shape and the working member (a vibration friction portion) has a complicated shape. However, the fifth patent document neither discloses nor teaches a length of the working member in a sliding direction.
With structure in the sixth patent document, it is difficult to shorten a length of the frictionally coupled portion between the driving shaft (the vibration friction portion) and the moving member (the moving portion) (i.e. a length of moving member in a sliding direction). It is therefore difficult to reduce the height (size) of the drive device.
In the driving apparatus disclosed in the seventh patent document, in the similar as a case of the drive device disclosed in the sixth patent document, the driving member (the vibration friction portion) comprises a driving shaft extending in the expansion/contraction direction of the electro-mechanical transducer and the moving member (the moving portion) comprises a slider which is frictionally in contact with the driving shaft. Therefore, in the similar manner as a case of the drive device disclosed in the sixth patent document, with the driving apparatus disclosed in the seventh patent document, it is difficult to shorten a length of the frictionally coupled portion between the driving member (the vibration friction portion) and the moving member (the moving portion) (i.e. a length of the slider in a sliding direction). The driving apparatus disclosed in the seventh patent document is disadvantageous in that it is difficult to reduce the height (size) of the driving apparatus. In addition, in the driving apparatus disclosed in the seventh patent document, the sandwiched member (the friction member) is pressed against the driving shaft (the vibration friction portion) by means of the elastic member (the leaf spring).
At any rate, any of the first through the seventh patent documents neither considers nor takes whether or not it is better what length is the frictionally coupled portion into consideration in order to rapidly move the moving member (the moving portion) with stability.